Appendix IV. 965 instantaneously in one flash. In other engines, like the original Benz, having but 800 revs, per m. or thereabouts, a trembling hammer on the coil causes a spark of longer duration. The magneto-machine is represented by the Sims-Bosch gear* shewn in Fig. 899. A is a permanent magnet of some strength, and B a fixed armature wound lengthwise with a wire c making circuit with the sparking plug E partly directly and partly through the engine frame D. F is a soft iron shield which is caused to rock on the armature centre by means of the connecting rod F coupled to a cam disc G on the engine shaft. As the cam rotates, it allows the tappet H to lower at the proper time and raise the trip lever j, thus breaking the armature circuit and causing the spark to pass at E within the cylinder. It is well known that the disturbance of a magnet's field will cause difference of potential in a circuit between its poles, and such disturbance is actually produced by the rocking of the soft iron shield. Fig. 900 shews the method of connecting up a small shunt- wound dynamo A, which is driven from the engine shaft. As the current cannot flow till the engine is running, the latter must be started by means of the secondary battery B, which is provided with a primary coil c having large self-induction; but as soon as the dynamo becomes excited by the engine it is switched in circuit with the coil and firing plug, while the accumulators are fed in parallel by a portion of the dynamo current, so that the cells are kept constantly charged. Thus by closing switch E, we connect the battery B, coil c, and firing plug; and when ready the dynamo is coupled by switch D, sending some of its current by c and F, and the overflow through B. When the cells are fully charged, E may be opened.